1. Field of the Invention
The present invention relates to an information recording apparatus and method capable of radiating a light beam onto an information recording medium to record multi-level data in a portion of the recording medium.
2. Description of the Related Art.
In order to increase the capacity and processing speed of computers in the future, an increase in the density and capability of memories is an important technical object. In currently available optical memories, such as optical disks and cards, only a one-level signal (i.e., a signal corresponding to the presence or absence of a data bit record) can be written in a single record spot. Mark-position recording and mark-edge recording are currently utilized to record information onto such optical memories. In mark-position recording, the center of the recording mark is positioned at the location of a data bit, whereas in mark-edge recording, the edge of recording mark is positioned at the location of a data bit. Generally, recording density is much higher in mark-edge recording than in mark-position recording. In both recording methods, however, only a one-level signal (i.e., a signal corresponding to the presence or absence of a data bit) can be written in a single record spot. Thus, if mark-edge recording is utilized, the recording density achieved is not sufficiently high. In order to overcome this limited memory density, a multi-value signal recording system for writing a multi-value signal in a single record spot is necessary. An ultra-high density optical memory using a photochemical hole burning (PHB) technique is an example of a memory employing such a multi-value recording system. See, for example, U.Itoh et al. Page 147 through 150 in Topical Meeting on Optical Data Storage, Mar. 11-13, 1987, Stateline, Nev. The PHB material has a host made of transparent materials and guest material dispersed in the host. Only the guest material absorbs light and a photochemical reaction occurs in the guest at a temperature as low as several degrees to several tens of degrees K. When a light beam having wavelength is radiated onto PHB material, the guest material absorbs the light beam so as to generate a hole of an absorption spectrum at wavelength .lambda.. By modulating wavelengths of light into n-levels and directing the light onto the guest material of PHB material, the guest material has n different absorption spectra, and n-value data can be recorded. When the recorded data is reproduced, the reflectivity or transmissivity of the recording medium is detected at the n-level wavelength. In such an n-value recording system using PHB, however, the PHB memory film must be kept at a very low temperature. Many problems concomitant with its use as an optical memory device remain unsolved. Hence, to date no practical applications of a high-density optical recording system have been realized.